Engineering problems related to mass and speed traditionally have been solved by providing ever larger sources of energy. As a result, relatively inefficient power conversion has become commonplace. In the field of space exploration, studies suggest that efficient robots of many designs are needed as precursors to manned exploration. Robots generally weight and cost less than manned space vehicles, and they do not require elaborate life support systems.
Piezoelectric actuators are among the devices being investigated for converting stored electric power to motive force in space vehicles. Piezoelectric actuators have advantages for use in remote and extreme environments because of their high efficiency and ability to operate on direct current without the need for lubrication or cooling. Detailed descriptions of piezoelectric actuators and electronic drive systems that produce piezoelectric "smooth walking" are set forth in the following co-pending U.S. Patent Applications, which are incorporated herein by reference: Ser. No. 252,197 filed Sep. 30, 1988, and Ser. No. 488,548 filed Mar. 5, 1990.
A chronic problem in some robotic joints is an effect termed slop. Slop encompasses phenomena such as gear backlash, bearing clearance play, elastic bending, and mechanical compliance that collectively result in failure of a robotic limb to extend to a position with the desired accuracy and steadiness. In space applications, robotic joints with minimal slop are needed in sizes ranging from large to microscopic. Electromagnetic actuators for robots are limited to relatively large embodiments because the surface area to volume ratio of miniaturized components increases dramatically, resulting in aggravated flux leakage. Furthermore, magnetic actuators dissipate power continuously in the form of current flow when maintaining a constant force.
In contrast to magnetic actuators, piezoelectric actuators are ideal for small scale applications. Electric field flux leakage does not increase dramatically as component size diminishes because piezoelectric layer thickness, and thus dielectric edge area, also decreases proportionally. The relative structural strength of piezoelectric actuators increases with decreasing size because of the importance of surface area to structural strength. Miniature components allow relatively higher stress levels, which improve specific performance. Furthermore, piezoelectric actuators apply a constant force once the appropriate amount of electric charge is stored in the piezoelectric material. As long as the charge persists, no further input of electric energy is required to maintain the applied force.
One of the preferred configurations for a walking robot comprises six legs with each leg having two joints. Efficient walking is the result of hundreds of millions of years of evolutionary development exemplified by the seemingly endless variety of six-legged insects. The most common mode of insect walking utilizes an alternating tripod. This mode is fast and stable as long as the center of gravity remains inside the triangles of both tripods. A one leg at a time mode can be used for scaling high obstacles. In this mode, the front legs reach upward while the rear two pair provide support and stability. This motion requires greater angular freedom of movement of the legs than is required for just walking on a level surface.
In the field of robotic vehicles for space exploration, there is a need for a robotic walking system suitable for dimensions ranging from microscopic to very large. The system must include flexible joints that can be actuated efficiently, reliably, and without slop.